In general, a differential pressure detection type pressure detector has a construction as stated below. The detector is disclosed in, for example, Japanese Patent Application Publication No. 2002-221462.
The pressure detector in the prior art includes a case which has a recess being open on one face side thereof, and it accommodates a pressure detection element in the recess. The pressure detection element is so arranged that its one face side confronts the open side of the recess, while its other face side confronts the bottom of the recess.
Besides, a penetrating hole or the like is provided in the bottom face of the recess. Thus, a pressure is exerted on one face of the pressure detection element from the open side of the recess, while another pressure is also exerted on the other face of the pressure detection element from the bottom face side of the recess. Further, a pressure is detected on the basis of the differential pressure between the pressures which are applied on one face and the other face of the pressure detection element.
Here, the recess, and the penetrating hole or the like provided on the bottom face side of the recess are filled up with a protective member of gel or the like. Thus, the pressure receiving faces of the pressure detection element, namely, one face and the other face thereof are covered with and protected by the protective member. Incidentally, the pressures are exerted on the pressure detection element through the protective member.
Meanwhile, in engine controls, etc., there are a large number of systems which detect pressures for the controls. In this regard, the pressure detector of the pertinent type is employed for the pressure detection.
Concretely, the pressure detector can be applied as one which is mounted on an EGR pipe being a member to-be-measured in the EGR system of the Diesel engine of an automobile vehicle, and which detects a differential pressure before and behind an orifice provided in the EGR pipe. Also, the pressure detector can be applied as one which is mounted on the exhaust pipe of the engine in order to detect the pressure loss of a DPF (Diesel particulate filter) disposed in the exhaust pipe, and which detects the differential pressure of the exhaust pipe before and behind the DPF.
In this manner, in an environment where the pressure detector is employed, a pressure medium to-be-measured contains, for example, a corrosive liquid, so that the pressure receiving faces of the pressure detection element need to be protected from the pressure medium. In the prior art, therefore, the pressure receiving faces of the pressure detection element is covered with and protected by the protective member as stated above.
Besides, in such an application as the engine control, the pressure detection element performs the pressure detection while undergoing the vibrations of the engine, etc. Especially in the application to the engine of the vehicle, the pressure detection element undergoes also the vibrations of the vehicle.
Here, in the case where the pressure receiving faces of the pressure detection element are covered with the protective member of gel or the like, stresses ascribable to the weight of the protective member itself act on the pressure receiving faces of the pressure detection element when the vibration is exerted on the pressure detector. Consequently, ripple noise ascribable to the vibration develops to be superposed on the output of the pressure detector.
Especially in a differential pressure detection type pressure detector which detects the differential pressure between the pressures of two pressure media, a pressure detection element has its one face and other face protected by locating protective members thereon, so that ripple noises ascribable to the protective members of both the faces are added up.
Concretely, the directions of the vibration are identical on one face and other face of the pressure detection element. Therefore, when the stress ascribable to the vibration has acted in the direction of pushing one face of the pressure detection element, the stress on the other face of the pressure detection element acts in the direction of pulling this other face.
Since the stresses act on the pressure receiving faces of the pressure detection element in this manner, the differential pressure detection type detector has involved the problem that the vibrational noise further increases to incur a large error, and that the pressure detector delivers the output of the error which is ascribable to the vibration differing from the acting pressures.
Here, it is also considered that, for the purpose of reducing the vibrational noises, the output of the pressure detector is filtered by a low-pass filter which removes output components at and below the vibrational frequencies of the noises. The filtering, however, gives rise to the demerit that pressure signal components above a filter frequency cannot be derived.